Carbon Budget and Cycling in Perennially Ice–Covered Lake Untersee, East Antarctica NICOLE MARSH Thesis submitted to the University of Ottawa in partial fulfillment of the requirements for the Master of Science Department of Earth and Environmental Science Faculty of Science University of Ottawa Under the supervision of: Dr. Ian Clark (Department of Earth and Environmental Science) Dr. Denis Lacelle (Department of Geography) © Nicole Marsh, Ottawa, Canada, 2019 Acknowledgements First, I want to thank my supervisors Dr. Ian Clark and Dr. Denis Lacelle. I’m very fortunate to have supervisors that are as knowledgeable on polar environments and as invested in this work as the both of you. It’s been an absolute pleasure working with and learning from you. Many thanks are owed to Dr. Dale Andersen (Carl Sagan Center, SETI Institute), who led the expedition to the Undersee Oasis and brought a team of scientists together for a grand adventure to one of the most extraordinary locations on Earth. Thank–you for your guidance in the field and with the science, and constant entertainment with your stories. I’d also like to thank the other expedition members for their help in the field and memories in South Africa; Benoit Faucher and Dr. Denis Lacelle (University of Ottawa), George Shamilishvily (St. Petersburg University, Russia), and Elliott Steele (University of Queensland, Australia). The expedition was possible due to funding from the TAWANI Foundation and the Trottier Family Foundation, logistical support by the Antarctic Logistics Centre International (ACLI), and collaboration with the Arctic/Antarctic Research Institute and Russian Antarctic Expedition (RAE). Funding for analytical work was generously provided by NSERC. I am grateful for the training, technical guidance and countless hours of analytical work performed by the lab technicians and collaborators at the University of Ottawa. Thank–you to Carley Crann, Sarah Murseli, Christabel Jean, Monika Wilk, and Dr. Xiaolei Zhao at the A.E. Lalonde AMS Laboratory, Paul Middlestead, Wendy Abdi, Patricia Wickham and Kerry Klassen at the Ján Veizer Stable Isotope Laboratory, and Smitarani Mohanty and Nimal De Silva at the Geochemistry Laboratory. Thanks to Sarina Cotroneo for her Sr–isotope work and Husnain Anwar for assisting with the lake cores. A special thanks to Benoit Faucher from whom I’ve received constant support in the field, laboratory and in scientific discussion. I feel very fortunate to have been apart of this project, and for the friends and family that supported me every step of the way. You have my love and gratitude. ii Abstract Perennially ice–covered lake Untersee is one of the largest (8.7 km2) and deepest (~160 m) freshwater lakes in East Antarctica. Water mass balance of Lake Untersee shows it receives ~ 45% of its annual input from melting of the glacial–wall beneath the ice–cover and ~55% from subglacial meltwater; with loss by sublimation of the ice–cover. The lake floor hosts active photosynthetic microbial mats despite weak irradiance through the ice–cover (<5% PAR). This study aims to characterize the carbon content and its isotopic composition (δ13C and 14C) in the lake–waters and microbial mats to develop a carbon budget in order to define carbon sources and its cycling in the lake ecosystem. The DIC in the oxic and alkaline water column (pH 10.4) is very 13 14 low and of atmospheric origin (0.3–0.4 ppm, δ CDIC = –7 to –10‰, F CDIC of 0.41 to 0.60). The organic–C content of microbial mats is 0.857 kg C m–2 and the surface layer has very similar δ13C (–9 to –12‰) to the DIC in the water column. The 14C ages of the top and bottom mat layers range from 9,524 to 10,052 years BP, with the age of the bottom mat layers (12,031–13,049 years BP) corresponding to the inferred timing of formation of the lake. Mass balance shows that the rate of the incoming carbon from both subglacial meltwater and englacial melting (8×104 g C y–1) is insufficient to account for the carbon sequestered by the microbial mats (4–8×109 gC). This suggests that Lake Untersee developed a summer moat when it initially developed (~12 to 13 kya), which allowed for open–exchange with atmospheric CO2 and replenishment of DIC in the water column. This is supported by a higher growth rate observed in the deepest microbial mats. Since 14 14 the permanent ice–cover developed, growth rate has decreased, and given the F CDIC and F CDOC in oxic waters (14C = 4,119 to 7,079 years BP), Lake Untersee has been well–sealed from atmosphere and the water–column subsequently became starved in carbon. These results demonstrate the capacity of microbial communities to adapt to harsh and shifting conditions in Earth’s most extreme environments. iii Contents Acknowledgements ......................................................................................................................... ii Abstract .......................................................................................................................................... iii Contents ......................................................................................................................................... iv List of Tables ................................................................................................................................ vii List of Figures .............................................................................................................................. viii 1 Introduction ............................................................................................................................. 1 1.1 Research Objectives ......................................................................................................... 2 2 Study Area ............................................................................................................................... 4 2.1 Site Location and Climate ................................................................................................ 4 2.2 Glacial History ................................................................................................................. 5 2.3 Geology and Surface Deposits ......................................................................................... 5 2.4 Lake Untersee ................................................................................................................... 7 2.5 Lake Water Chemistry ..................................................................................................... 7 2.6 Lake Ecosystem ................................................................................................................ 8 2.7 Lacustrine Sediments ..................................................................................................... 10 Chapter 2 Figures .................................................................................................................... 11 3 Methodology .......................................................................................................................... 16 3.1 Field Logistics ................................................................................................................ 16 3.2 Field Measurements and Sampling ................................................................................ 17 3.3 Laboratory Analyses ...................................................................................................... 19 3.3.1 Major Ions, Rare Earth Elements and Trace Metals in Waters ............................... 19 3.3.2 Carbon Content and δ13C in Waters ....................................................................... 20 iv 3.3.3 Radiocarbon Analysis of TIC and TOC in Waters ................................................. 20 3.3.4 Strontium (87Sr/86Sr) Isotopes in Waters ................................................................. 22 3.3.5 Sulfur Isotopes of Sulfate and Sulfide in Waters .................................................... 23 3.3.6 Nitrogen and Oxygen Isotopes of Nitrate in Waters ............................................... 24 3.3.7 Tritium (3H) in Waters ............................................................................................ 25 3.3.8 Radioiodine (129I) in Waters.................................................................................... 25 3.3.9 Microbial Mats Sample Preparation ....................................................................... 26 3.3.10 Organic–Carbon Content and δ13C of Microbial Mats ........................................... 27 3.3.11 Radiocarbon Measurements of Microbial Mats ...................................................... 28 3.4 Carbon Budget ................................................................................................................ 29 Chapter 3 Tables ..................................................................................................................... 31 Chapter 3 Figures .................................................................................................................... 32 4 Results ................................................................................................................................... 36 4.1 North Basin Water Column ............................................................................................ 36 4.2 South Basin Water Column ............................................................................................ 38 4.3 Moraine Ponds...............................................................................................................